Project Summary In pathogenic HIV and SIV infection innate and adaptive immune responses fail to control viral replication and, coupled with persisting immune activation, lead to the progressive depletion of CD4+/CCR5+ T cells and AIDS. Numerous HIV and SIV vaccines have also failed to elicit immune responses that can broadly prevent or control infection. Although immune control of HIV and SIV can occur, the determinants are unclear, as is the ability to induce these responses with vaccines. There is thus, an urgent need to identify and understand immune responses that can broadly prevent or control HIV/SIV infection and to elicit these responses with vaccines. A model is described in which deletion of a Gly and Tyr within a highly conserved trafficking motif in the SIVmac239 envelope (Env) cytoplasmic tail produces a virus (?GY) that in pigtail macaques (PTM) replicates acutely to wildtype levels, but becomes highly controlled by cellular immune responses in the absence of neutralizing antibodies. Remarkably, animals that control ?GY infection are able to prevent or control diverse challenges with homologous SIVmac239, heterologous SIVsmE660 and SHIV-SF162P3N, which are all highly pathogenic in PTM. Recent studies of mutations acquired in rare ?GY animals that progressed to AIDS and of viral evolution in PTM depleted of CD8 cells prior to ?GY infection, have implicated the loss of polarized trafficking of Env in infected cells as a driver for ?GY's altered pathogenesis. These findings have suggested that Env trafficking plays a critical dual role in promoting both cell-cell spread of virus in tissues and in evading CD8 cellular immune responses. As these events are mediated by virologic and immunologic synapses, respectively, we hypothesize that this motif is critical in vivo in promoting infectivity and modulating viral susceptibility to cellular immune attack. Findings with the ?GY model suggest that disrupting this motif can not only alter pathogenesis but also enable potent and broadly protective immune responses to occur. This testable hypothesis and its implications for vaccines will be assessed in 4 Aims: 1) To identify the immune correlates of ?GY control and responses that are shared during control of diverse challenges; 2) To provide a mechanistic understanding of this model by characterizing differences in virologic and immunologic synapses during ?GY and SIVmac239 infection; 3) To assess the impact of alterations in Env trafficking on immunogenicity in the context of a novel mRNA vaccine; and 4) To extend findings for the immunomodulatory role of this conserved trafficking motif in SIV to an HIV-1 Env. These highly novel observations are positioned to provide new insights into protective immunity for the SIV and HIV vaccine fields.